Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation on August 16, 2007.
Basis for Assessment
SUMMARY: The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to high levels of pH that exceed Iowa’s Class B(LW) criterion. A fish kill on August 16, 2007 also suggests impairment of the Class B(LW) uses. However, in terms of nutrient concentrations, water clarity, and algae populations monitoring data suggest excellent water quality at this lake. Fish consumption uses remain “not assessed” due to a lack of information on which to base an assessment. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation on August 16, 2007.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class B(LW) uses at Moorehead Lake are “partially supported" due to violations of the Class B(LW) criterion for pH. Results of the ISU and UHL lake surveys from 2002-2006 show no violations of the Class B(LW) criterion for ammonia in 17 samples and no violations of the Class B(LW) criterion for dissolved oxygen in 23 samples. However, there were 5 violations of the Class B(LW) criterion for pH in 23 samples (22%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the Class B(LW) uses of Moorehead Lake.
Using the median values from the ISU and UHL lake surveys from 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 49, 52, and 57 respectively for Moorehead Lake. According to Carlson (1977) the chlorophyll a and total phosphorus values place Moorehead Lake in the eutrophic category, while the Secchi depth value places Moorehead Lake in the mesotrophic category. These values suggest low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and relatively low levels of phosphorus in the water column.
The level of inorganic suspended solids is low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Moorehead Lake was 2.0 mg/L, which was the 22nd lowest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a small population of cyanobacteria exists at Moorehead Lake, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 53% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (3.7 mg/L) was the 17th lowest of the 132 lakes sampled.
Results of a fish kill investigation that occurred on August 16, 2007 also suggest “partial support” of the Class B(LW) uses of Moorehead Lake. The fish kill was attributed to low oxygen conditions caused by an algae die-off (natural conditions/non-pollutant). The number and type of fish killed was not reported. According to the IDNR assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2004-2007) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired.” If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
